Ink jet printing is a non-impact method that produces droplets of ink which are deposited on a substrate such as paper or transparent film in response to a digital signal. Ink jet printers, especially thermal or bubble jet drop-on-demand printers, have found broad application as output for personal computers in the office and the home.
Thermal ink jet printers use a plurality of nozzles, each containing a resistor element, to fire ink droplets toward the print media. Nozzle openings are typically about 40-60 micrometers in diameter. For proper operation of the printer, it is imperative that the ink jet ink not clog or plug these small openings. For pigmented ink jet inks in particular, it is necessary that the pigment particles be of small enough particle size so as to not clog the ejection orifice in the nozzle. Small pigment particles are also advantageous because they are less prone to settling during storage.
Pigment particles start in an agglomerated or flocculated state. Thus, it is necessary to disperse and stabilize the pigment so as to prevent flocculation and settling. The quality of the pigment dispersivity may also affect some ink jet printing characteristics such as ejectability, print quality, optical density, etc.
Ink jet inks have been made using a variety of different dispersion processes. U.S. Pat. No. 5,026,427 teaches the use of a liquid jet interaction chamber in the preparation of pigmented ink jet inks. U.S. Pat. No. 5,085,698 teaches the preparation of aqueous pigmented inks for ink jet printers using a media mill, a ball mill, an attritor, or a liquid interaction chamber. U.S. Pat. No. 4,597,794 teaches the preparation of pigmented ink jet inks by ball mill, roll-mill, speed line mill, homomixer, sand grinder, and the like.
T. C. Patton, "Paint Flow and Pigment Dispersion", John Wiley & Sons, N.Y., N.Y., p.386 (1979) discloses that a variety of different processes can be used to disperse pigments. These include ball and pebble mills, high speed disk impellers, high speed impingement mills, three-roll mills, high speed stone and colloid mills, sand mills, and batch attritors.
The term "roll mill" as used in the ink industry, refers to a three-roll milling apparatus. See, for example, Patton at p. 388:
The three-roll mill . . . is representative of the roller mills used by the coating and ink industries to grind (disperse) pigments into vehicles. Four-roll and five-roll mills are also manufactured, but except in ink preparation this extension of the three-roll principle has not taken hold in the paint industry. PA1 The roller mills--also called roll mills--which have been most commonly used in paint grinding are triple roll mills and single roll mills, although mills with up to five rollers are available.
See also Nylan and Sunderland, "Modern Surface Coatings", John Wiley & Sons, N.Y., N.Y., p 529 (1979):
Two-roll mills, on the other hand, have been widely used in the rubber industry (hence the name "two-roll rubber mills") to masticate rubbers, to blend in various components (e.g. antioxidants) or to mix different types of rubbers together. Such mills typically comprise a pair of horizontally disposed, spaced-apart, parallel, stainless steel cylindrical rolls which rotate inwardly toward one another. The distance between the rolls, known as the "gap," is variable, as is the temperature and rotational speed of each roll independently.
After much research and experimentation, the present Applicants have discovered that pigmented ink jet inks prepared from dispersions made using a two-roll mill have improved properties as compared to other dispersion techniques heretofore used to prepare ink jet inks, including three-roll mills or "roll mills."